Project Summary The HIV host restriction factor APOBEC3G (A3G) can inhibit HIV by inducing catastrophic mutations in the viral genome. The HIV Vif protein protects HIV by binding to A3G and directing it to the proteasomal degradation pathway. The significance of the proposed SBIR phase I research is that Vif remains a new antiviral target whose clinical potential has yet to be fully explored. Despite much academic research on Vif, only a limited effort has gone toward identifying small molecule antagonists of Vif. OyaGen, Inc. is the only commercial entity currently pursuing the identification of chemical scaffolds that specifically interact with Vif in meaningful ways for drug development. An inherent limitation of prior anti-Vif drug discovery efforts has been the use of primary drug screens in which ?hits? may be due to more than one mechanisms of action. We designed a live cell quenched FRET (FqRET) reporter assay that enables selection of membrane- permeable antagonists of protein-protein interactions between Vif and A3G using a Vif mutant that retains wild type binding to A3G but no longer binds to Elongin C and no longer facilitates A3G degradation. From a screen of a 110K small molecule library of drug-like compounds we identified 165 prioritized his for further analysis. Although a few compounds were dose-dependent antagonists of Vif binding of A3G, one compound, hereafter referred to as C5, was non-cytotoxic and displayed dose-dependent: i) inhibition of Vif-dependent A3G degradation, ii) inhibition of pseudotyped HIV replication and iii) promoted an increased incorporation of A3G in virions. Here we propose an innovative drug discovery critical path to optimize a novel anti-HIV lead compound from C5 in anticipation of phase II SBIR pharmacokinetic analysis, in vivo efficacy testing, and IND-enabling studies. The Phase I SBIR Aims will focus on structure-activity relationship (SAR) medicinal chemistry of the C5 scaffold for hit-to-lead optimization of anti-HIV compound whose antiviral activity is mechanistically due to its function as a Vif antagonist that protects A3G from Vif-dependent degradation.